Hydrocarbons, such as oil and gas, may be recovered from various types of subsurface geological formations. The formations typically consist of a porous layer, such as limestone and sands, overlaid by a nonporous layer. Hydrocarbons cannot rise through the nonporous layer, and thus, the porous layer forms a reservoir in which hydrocarbons are able to collect. A well is drilled through the earth until the hydrocarbon bearing formation is reached. Hydrocarbons then are able to flow from the porous formation into the well.
In what is perhaps the most basic form of rotary drilling methods, a drill bit is attached to a series of pipe sections referred to as a drill string. The drill string is suspended from a derrick and rotated by a motor in the derrick. A drilling fluid or “mud” pumped down the drill string, through the bit, and into the well bore. This fluid serves to lubricate the bit and carry cuttings from the drilling process back to the surface. As the drilling progresses downward, the drill string is extended by adding more pipe sections.
When the drill bit has reached the desired depth, larger diameter pipes, or casings, are placed in the well and cemented in place to prevent the sides of the borehole from caving in. Once the casing is cemented in place, it is perforated at the level of the oil bearing formation so hydrocarbons can enter the cased well. If necessary, various completion processes are performed to enhance the ultimate flow of hydrocarbons from the formation. The drill string is withdrawn and replaced with production tubing. Valves and other production equipment are installed in the well so that the hydrocarbons may flow in a controlled manner from the formation, into the cased well bore, and through the production tube up to the surface for storage or transport.
That simplified example of an oil and gas well, comprising as it does a single casing and a single tube, is not often encountered in the real world. Given the depth of most producing oil and gas wells and various environmental considerations, they more commonly incorporate a number of pipes or “tubulars” of varying diameters. Casings of diminishing diameter may be “telescoped” together to extend the depth of the well. There may be several production zones and multiple production strings, and it usually is necessary to run surveying and logging equipment into a well to assess the formation. In short, there are a wide variety of tools and operations that must be completed successfully in order to construct and operate a typical oil or gas well.
It is not surprising, therefore, that not all of the required operations are completed successfully. Accidents happen. Work strings break. Tools get jammed and must be drilled out. Things fall into wells. Such objects may have to be retrieved or “fished” out of a well before normal operations may be resumed, and doing so can create even more junk. Larger objects or “fish” may have to be ground or broken into smaller pieces so that they may be grabbed more easily. Explosives also may be used to break up a large fish. Even in the absence of such operations, however, cement lumps, rocks, congealed mud, metallic scale and shavings, and other debris may collect near the bottom of a well to a degree that it must be removed before production tubing may be installed.
The particular type of fishing tool employed depends in large part on the type of objects or “fish” to be retrieved from a well. So-called “junk baskets,” such as those disclosed in U.S. Pat. No. 4,084,636 to E. Burge and U.S. Pat. No. 5,944,100 to J. Hipp, are adapted for retrieval of smaller pieces of junk and debris. Junk baskets typically rely on circulation of drilling fluid to sweep debris into a trap. The trap often includes hinged fingers which swing inwardly to allow debris to wash into the trap and then swing back out to close the trap. They also may incorporate teeth at the bottom of the tool for milling or grinding larger fish. While they can be effective in certain situations, such tools may be fairly complex, requiring as they typically do various channels, valves, and other fluid control mechanisms designed to create a flow of drilling fluid sufficiently powerful to sweep relatively dense debris upward into a trap.
So-called “washover” retrieval tools also have been designed to fish debris and smaller objects out of a well. Examples of such tools are disclosed in U.S. Pat. No. 4,545,432 to R. Appleton and U.S. Pat. No. 7,992,636 to G. Telfer. While there are certain differences, those tools share a common basic design. The generally cylindrical main housing of the tool defines a chamber, the lower end of which is open. Cutting teeth may be provided around the lower periphery of the open end of the housing. The tool housing also defines an annular space between inner and outer walls of the housing. That annular space essentially serves as a hydraulic cylinder in which is mounted an annular piston. The annular piston has malleable fingers at its lower end which are closed to entrap debris in the chamber.
More specifically, the tools are operated first by rotating the tool to drill through and under debris, for example, debris that has collected in the bottom of a well. That typically is done using reverse fluid circulation to encourage debris to flow into the chamber. Once drilling is complete, fluid is pumped down a work string into the tool and into the annular cylinder to actuate the piston. As the piston travels downward, the fingers at its lower end impinge on a mill surface provided on the lower end of the tool housing. Continued downward travel of the piston deforms and shapes the fingers into a basket, closing off the bottom of the chamber and preventing debris collected therein from falling out as to tool is pulled from the well.
While not without certain advantages, washover tools of this type can be problematic. The tool frequently is operated with reverse fluid circulation, that is, while fluid is drawn into the tool instead of being pumped out of the tool. Reverse circulation is intended to sweep cuttings and other debris into the tool, but that debris can interfere with actuation of the piston. If debris lodges on a valve seat, for example, it may not be possible to build up sufficient pressure to actuate the piston. Moreover, the tools rely on a shear disc, blowable valve seat, or other pressure limiting device to signal when the piston has fully stroked and the basket has been completely closed. If the piston hangs up in the cylinder, however, pressure may build to the point that the pressure limiting device is actuated before the basket has closed. Once that happens, there is no mechanism for completing the piston's stroke and closing the basket.
The annular cylinder also may be susceptible to damage, comprising as it does the external wall of the tool housing, especially if the tool is used to drill under debris. Any damage to the cylinder may increase the likelihood that the piston will hang up during operation. Providing a sufficiently rugged annular cylinder in the housing wall, other factors being equal, also necessarily diminishes the “swallow” diameter of the tool, that is, the diameter of the opening and chamber which accommodates a fish or other debris.
Such washover tools also are somewhat limited in their ability to handle large fish that may not be completely swallowed by the tool. They also may encounter problems if the tool has drilled into hard formation at the bottom of a well. The malleable fingers which are shaped into a basket during actuation of the tool typically are fabricated from a relatively soft metal such as aluminum. Thus, they are limited in their ability to cut or drive through any material that may be in their path as the tool is actuated. If metallic fish or other hard materials are present in the tool opening, it may not be possible to shape the malleable fingers into a basket, or the fingers may hang up on the partially enveloped fish. Both scenarios may make it more difficult or impossible to retrieve material from the well.
Accordingly, there remains a need for new and improved systems, apparatus and methods for retrieving junk and other debris in oil and gas wells. Such disadvantages and others inherent in the prior art are addressed by various aspects and embodiments of the subject invention.